High voltage circuit

ABSTRACT

A high voltage generation circuit comprising a transformer, the transformer having a primary winding having a primary inductance; a power supply for supplying a voltage to the primary winding of the transformer; a switching element for controlling current flowing through the primary winding of the transformer from the power supply; and a resonance capacitor which resonates with the primary winding of the transformer when the switching element is off, so that a flyback pulse is generated. The circuit has a distributed inductance and a resonance capacitance. The switching element is controlled so as to be switched on nearly at the bottom of a quiescent ringing pulse which is produced by a resonance of the primary inductance of the transformer with a capacitance included in a circuit connected to the primary winding of the transformer, after the flyback pulse is generated.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a high voltage generationcircuit and, more particularly, to a high voltage generation circuit forgenerating a high voltage to be supplied to a CRT (cathode ray tube) orthe like.

[0003] 2. Description of the Related Art

[0004]FIG. 1 is a circuit diagram showing an example of a high voltagegeneration circuit as the background of the present invention. The highvoltage generation circuit 10 includes a transformer 12. The primarywinding of the transformer 12 is connected to the anode of a diode 14.The cathode of the diode 14 is connected to the drain of FET 16 as aswitching element. The source of the FET 16 is connected to a resistor18. The other side of the resistor 18 is grounded. A diode 20 isconnected in parallel to the series circuit comprising the diode 14, theFET 16, and the resistor 18. The cathode of the diode 20 is connected tothe anode side of the diode 14. The anode of the diode 20 is grounded.

[0005] A series circuit comprising a resonance capacitor 22 and a diode24 is connected in parallel to the diode 20. One end of the resonancecapacitor 22 is connected to the anode side of the diode 14, and theother end of the resonance capacitor 22 is connected to the cathode ofthe diode 24. The anode of the diode 24 is grounded. Moreover, the nodebetween the resonance capacitor 22 and the diode 24 is connected to theanode of another diode 26. The cathode of the diode 26 is connected tothe primary winding of the transformer 12 via a ringing suppressioncircuit 28. The ringing suppression circuit 28 comprises a capacitor 30,a resistor 32, and an inductor 34. A power supply +B is connectedbetween the diode 26 and the ringing suppression circuit 28. The nodebetween the diode 26 and the ringing suppression circuit 28 is groundedvia a capacitor 36 and an electrolytic capacitor 38.

[0006] To the gate of the FET 16, a signal for on and off controlthereof is provided from a PWM (Pulse Width Modulation) control circuit40. Voltage produced by dividing a secondary output voltage of thetransformer 12 is input to the PWM control circuit 40. This voltage anda horizontal driving signal are input to the PWM control circuit 40. ThePWM control circuit 40 generates a control signal for controlling theFET 16. A node between the FET 16 and the resistor 18 is connected to aprotection circuit provided in the PWM control circuit 40, so that anover-current flowing in the circuit is detected.

[0007]FIG. 2 shows waveforms at the respective portions of the highvoltage generation circuit 10. FIGS. 2(a), (b), and (c), respectivelyrepresent the waveform chart of a signal for controlling the FET 16, thevoltage at point A shown in FIG. 1, and the current flowing through theprimary winding of the transformer 12. First, when the FET 16 is turnedon at t₀, current flows from the power supply +B through the diode 14,the FET 16, and the resistor 18. Electromagnetic energy is stored in theprimary winding of the transformer 12, due to the current.

[0008] The FET 16 is turned off at t₁. At this time, current flows fromthe primary winding of the transformer 12 through the resonancecapacitor 22 and the diode 26, and the primary winding of thetransformer 12 and the resonance capacitor 22 start to resonate. Asshown in the waveform chart of FIG. 2(b), a flyback pulse is generated.The flyback pulse becomes maximum when all of the electromagnetic energystored in the transformer 12 is converted to electrostatic energy of theresonance capacitor 22.

[0009] After all of the electromagnetic energy stored in the primarywinding of the transformer 12 is transferred to the capacitor 22,reverse current flows through the diode 24, the resonance capacitor 22,and the primary winding of the transformer 12. Thus, the electrostaticenergy in the resonance capacitor 22 is reversely converted to theelectromagnetic energy in the primary winding of the transformer 12. Atthis time, the diode 14 prevents electric charge stored in the parasiticcapacitance of the FET 16 from flowing out toward the primary windingside.

[0010] At t₂ when the flyback pulse is completed, the potential at thepoint A becomes zero. Then, the diode 20 is turned on, so that currentflows from the ground side of the diode 20 into the primary winding ofthe transformer 12. The current increases the voltage at the point A.The voltage at the point A has the same potential as that of the powersupply+B at t₃. At this time, the diode 20 is turned off, and thecurrent becomes zero. Then, as regards the flow of current from thepower supply+B into the resonance capacitor 22, the potential at bothends of the resonance capacitor 22 is clamped to the voltage of thepower supply +B by a current-blocking clamp circuit comprising thediodes 24 and 26, so that no current flows from the primary winding ofthe transformer 12 into the resonance capacitor 22. Then, the FET 16 isturned on at t₄, so that current flows from the power supply +B towardthe primary winding, and the circuit returns to the initial state at to.This operation is repeated. Thus, the circuit operation is continued.Accordingly, the voltage of the flyback pulse is increased by thetransformer 12, so that high voltage is output from the secondarywinding.

[0011] Capacitances included in the circuit, such as the parasiticcapacitance in the FET 16, exist at t₃ when the current becomes zero.Accordingly, resonance with the primary winding of the transformer 12occurs, and a quiescent ringing pulse is generated during the time fromt₃ to t₄. The ringing suppression circuit 28 is used to suppress theringing vibration pulse.

[0012] In the high voltage generation circuit 10, the primary inductanceLp of the transformer 12 is designed so as to satisfy the condition ofLp≦Eb·Ts/Ipp in which Eb is a source voltage, Ts is the time from thecompletion of a flyback pulse to the start of the next flyback pulse,and Ipp is the allowed current of the FET 16. Conventionally, such ahigh voltage generation circuit is designed such that theabove-mentioned condition is satisfied, and a required output voltagecan be obtained from the secondary winding of the transformer 12.

[0013] However, if the FET is turned on nearly at the peak of thequiescent ringing pulse as shown in FIG. 3, the high voltage of thequiescent ringing pulse is instantaneously terminated. Thus, the ringingwhich is determined by the distributed capacitance of the transformer 12and so forth is generated, so that overshoot and undershoot occur incurrent flowing through the primary winding of the transformer 12. Thegeneration of such overshoot and undershoot causes a problem in thatlosses in the transformer 12 and a resistance loss in the ringingsuppression circuit are increased.

SUMMARY OF THE INVENTION

[0014] Accordingly, it is a principal object of the present invention toprovide a high voltage generation circuit in which the loss caused byovershoot and undershoot when the switching element is turned on can bereduced.

[0015] According to the present invention, there is provided a highvoltage generation circuit which comprises a transformer, a power supplyfor supplying power to the primary winding of the transformer, aswitching element for controlling current flowing through the primarywinding of the transformer from the power supply, and a resonancecapacitor which resonates with the primary winding of the transformerwhen the switching element is off, so that a flyback pulse is generated,the switching element being controlled so as to be switched on nearly atthe bottom of a quiescent ringing pulse which is produced by theresonance of the inductance of the primary winding of the transformerwith the capacitance included in a circuit connected to the primarywinding of the transformer, after the flyback pulse is generated.

[0016] In the high voltage generation circuit, preferably, the controlof the switching element is carried out by adjusting at least one of theprimary inductance of the transformer, the distributed inductance, thevoltage of the power supply, and resonance capacitance.

[0017] Since the timing at which the switching element is turned on inthe high voltage generation circuit is controlled so as to occur nearlyat the bottom of the quiescent ringing pulse, the quiescent ringingpulse is terminated in the low voltage portion thereof. Accordingly,overshoot and undershoot is suppressed from generating in the currentflowing through the primary winding of the transformer, and losses inthe transformer and the ringing suppression circuit can be reduced.

[0018] As seen in the above description, controlling the timing at whichthe switching element is turned on so as to coincide with the bottom ofthe quiescent ringing pulse or its neighborhood can be made by adjustingthe primary inductance of the transformer, the distributed inductance,the voltage of the power supply, the resonance capacitance, and thelike.

[0019] The above-described and other objects, features and advantages ofthe present invention will be apparent from the following detaileddescription of the preferred embodiments of the invention in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0020]FIG. 1 is a circuit diagram of an example of a high voltagegeneration circuit as the background of the present invention;

[0021]FIG. 2(a) shows a signal for controlling FET of FIG. 1, FIG. 2(b)shows voltage at point A of FIG. 1, and FIG. 2(c) shows current flowingthrough the primary winding of the transformer of FIG. 1;

[0022]FIG. 3 is a waveform chart of current flowing through the primarywinding when the FET is turned on nearly at the top of a quiescentringing pulse;

[0023]FIG. 4 illustrates a relation between the quiescent ringing pulseand the current flowing through the primary winding when the primaryinductance of the transformer is changed; and

[0024]FIG. 5 is a waveform of current flowing through the primarywinding when the FET is turned on nearly at the bottom of the quiescentringing pulse.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0025] In the high voltage generation circuit 10 shown in FIG. 1, theprimary inductance Lp of the transformer 12 has a maximum control limitin the PWM control system, expressed by the condition of Ipp=Eb·Ts/Lp inwhich Eb is the power supply voltage, Ts is the time from the completionof a flyback pulse to the start of the next flyback pulse, and Ipp isthe allowed current of the FET 16. Therefore, the high voltagegeneration circuit 10 to be operated in the PWM control system isdesigned so as to satisfy the condition of Lp≦Eb·Ts/Ipp. Moreover, thetime at which the FET 16 starts to switch on can be set to coincidesubstantially with the bottom of the quiescent ringing pulse bycontrolling the primary inductance of the transformer 12, thedistributed inductance, the voltage of the power supply +B, theresonance capacitance, and so forth.

[0026] For example, the primary inductance of the transformer 12 can becontrolled by adjustment of the number of turns of the transformer 12.The current flowing through the primary winding when the primaryinductance Lp of the transformer 12 is changed will be discussed. Theslope of the current waveform caused when the FET 16 is on is defined byΔIpp/Δt. Here, the allowed current Ipp is Ipp=Eb·t/Lp. Thus, the slopeof the current flowing through the primary winding of the transformer 12can be expressed as Eb/Lp. Accordingly, when the primary inductance Lpof the transformer 12 is adjusted from Lp1 to Lp2 (Lp2>Lp1), the slopeof the current waveform is reduced. Thus, the timing at which the FET 16is turned on can be made earlier. By adjusting the primary inductance Lpof the transformer 12 as described above, the time at which the FET 16starts to be turned on can be controlled. Thus, the time at which theFET 16 starts to be turned on can be made to coincide substantially withthe bottom of the quiescent ringing pulse.

[0027] Moreover, the on-start time of the FET 16 may be controlled so asto coincide substantially with the bottom of the quiescent ringing pulseby adjusting the resonance capacitance of the circuit connected to theprimary winding of the transformer 12. Thus, the on-start time of theFET 16 may be controlled, or the time when the quiescent ringing pulseis generated may be controlled. Any manner may be employed, providedthat the on-start time of the FET 16 can be made to coincidesubstantially with the bottom of the quiescent ringing pulse.

[0028] As shown in the above description, by turning on the FET 16nearly at the bottom of the quiescent ringing pulse, the quiescentringing pulse can be terminated when the voltage is in the low state.Therefore, substantially no overshoot or undershoot in the waveform ofcurrent flowing through the primary winding of the transformer 12 isgenerated when the FET 16 is turned on as shown in FIG. 5. Accordingly,losses in the transformer 12 and in the ringing suppression circuit 28,which may be caused by the overshoot or undershoot, can be suppressed.Moreover, the whole power consumption of the high voltage generationcircuit 10 can be reduced. These effects can be also obtained for a highvoltage generation circuit excluding the clamping circuit comprising thediodes 24 and 26.

[0029] In the high voltage generation circuit according to the presentinvention, the overshoot or undershoot of current flowing through theprimary winding of the transformer can be suppressed, and thereby,losses in the transformer and in the ringing suppression circuit can bereduced. Accordingly, the power consumption of the high voltagegeneration circuit can be decreased.

[0030] Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art.Therefore, the present invention should be limited not by the specificdisclosure herein, but only by the appended claims.

What is claimed is:
 1. A high voltage generation circuit comprising: atransformer, the transformer having a primary winding having a primaryinductance; a power supply for supplying a voltage to the primarywinding of the transformer; a switching element for controlling currentflowing through the primary winding of the transformer from the powersupply; and a resonance capacitor which resonates with the primarywinding of the transformer when the switching element is off, so that aflyback pulse is generated; the circuit having a distributed inductanceand a resonance capacitance; the switching element being controlled soas to be switched on nearly at the bottom of a quiescent ringing pulsewhich is produced by a resonance of the primary inductance of thetransformer with a capacitance included in a circuit connected to theprimary winding of the transformer, after the flyback pulse isgenerated.
 2. The high voltage generation circuit of claim 1, whereinthe control of the switching element is made by adjusting at least oneof the primary inductance of the transformer, the distributedinductance, the voltage of the power supply, and the resonancecapacitance.
 3. A method for controlling the on-timing of a switchingelement of a high voltage generation circuit, the circuit comprising: atransformer, the transformer having a primary winding having a primaryinductance; a power supply for supplying a voltage to the primarywinding of the transformer; a switching element for controlling currentflowing through the primary winding of the transformer from the powersupply; and a resonance capacitor which resonates with the primarywinding of the transformer when the switching element is off, so that aflyback pulse is generated; the circuit having a distributed inductanceand a resonance capacitance; the method comprising: controlling theswitching element so as to switch the switching element on nearly at thebottom of a quiescent ringing pulse which is produced by a resonance ofthe primary inductance of the transformer with a capacitance included ina circuit connected to the primary winding of the transformer, after theflyback pulse is generated.
 4. The method of claim 3, wherein the stepof controlling the switching element comprises adjusting at least one ofthe primary inductance of the transformer, the distributed inductance,the voltage of the power supply, and the resonance capacitance.